The New South Wales Rural Fire Service (RFS) says at least 33 homes have been lost in a bushfire west of Coonabarabran, which also damaged the Siding Spring Observatory.

The fire in the Warrumbungle National Park in the north-west of the state has burnt out nearly 40,000 hectares and has a 100-kilometre-wide front.

About 100 people living in the area have been forced to evacuate their homes as officials warn the amount of properties lost could rise.

A watch-and-act alert has been issued for the fire which is burning in a northerly direction about one kilometre south of Bugaldie.

Inspector Brett Lachlan from the RFS says properties to the west of the Warrumbungle National Park may come under threat if firefighters cannot contain the fire.

"We have a large number of fire trucks obviously at Bulgaldie, prepared for whatever might eventuate," he said.

"Finalise property preparations and things like that, it is still a very dynamic fire, the situation's going to continue to evolve."

Most residents have been evacuated to Baradine and Coonabarabran.

The RFS has confirmed 33 properties and more than 50 sheds have been destroyed, as well as machinery and there have been extensive stock losses.

The NSW Government has launched a website with information for affected communities. Click here

RFS Deputy Commissioner Rob Rogers says the fire burned on Sunday with a ferocity not seen for many years in New South Wales.

We know people are hurting, we know people have lost homes and we know homes are more than bricks and mortar.

We know it's very important... but not as important as their life.

RFS Deputy Commissioner Rob Rogers

"The smoke plume of that fire extended some 14km into the air and even prevented us from getting aircraft overhead because of just how dangerous and bumpy the conditions were," he said.

"Embers were being blown ahead of the fire and starting a new fire some 5km ahead and it became very apparent early in the piece that there was just absolutely no stopping that fire and it was simply too unsafe to leave people."

Mr Rogers says those who were removed from the area should not try to return until authorities deem it safe.

"At this point in time, we ask for their patience," he said.

"We know people are hurting, we know people have lost homes and we know homes are more than bricks and mortar, we know that it's their possessions and memories, we know it's very important... but not as important as their life."

Mr Rogers says crews are throwing every resource at the fire but he cannot guarantee it will be contained before warm weather returns later in the week.

Meanwhile, a watch and act alert has been issued for another fire, north-east of Coonabarabran, which has closed the Newell Highway.

The RFS says the two-hectare blaze is burning out of control but is not threatening properties at this stage.

And a fire near Eugowra, east of Forbes, which jumped containment lines this afternoon, has been brought back under control.

There were concerns it would reach the nearby Nangar National Park but crews are working to strengthen containment lines.

Eighteen staff from the Siding Spring Observatory had to be led to safety at the height of the blaze last night.

The observatory, run by the Australian National University (ANU), is the country's largest optical astronomy research facility and was deliberately located in the Warrumbungle Ranges for the altitude, clear air, and low humidity.

ANU acting vice-chancellor Erik Lithander says five buildings, including cottages that house staff, have been destroyed, but fortunately the facility's 15 telescopes remain intact.

"First and foremost, the major relief is that all the staff were brought safely down from the mountain," he said.

"The fact that the telescopes seem to be undamaged is significant because this is the largest observatory site in Australia - it's the home to the largest optical telescope in Australia."

The university says it has about $80 million worth of assets on the site and they are fully covered by insurance.

Dr Lithander says there will be closer inspection of the site when it is deemed safe.

"We do not yet know that impact the extreme heat and the ash has had on the telescopes themselves, and we won't be able carry out that assessment until we can enter the buildings and inspect the inside of them," he said.

ANU staff are travelling from Canberra to take stock of the fire's impact.

The astronomer in charge of the observatory, Professor Fred Watson, has been monitoring the situation from Norway.

He said lessons were learnt from the fires which destroyed the Mount Stromlo observatory in Canberra in 2003.

"For example there was a program of undergrowth clearing. There was a program of fitting ember screens to all the windows and all the buildings on the observatory, which means that you can't get embers penetrating into the buildings and setting fire from the inside, which is what happened at Stromlo," he said.